1. Field of the Invention
The present invention relates to an exhaust device for an internal combustion engine for discharging an exhaust gas from the internal combustion engine through a multistage expansion type muffler having a plurality of expansion chambers for silencing the sound of the exhaust gas during the passage thereof.
2. Description of Background Art
A multistage expansion type muffler is know that includes a plurality of expansion chambers, a pipe for communicating between the expansion chambers, and a tail pipe, wherein the pressure of an exhaust gas from an internal combustion engine is reduced in each expansion chamber to thereby silence the sound of the exhaust gas. The exhaust gas, that is reduced in pressure, is discharged from the tail pipe to the outside of the muffler. See, for example, Japanese Patent Laid-open No. Hei 01-285615.
In the above conventional exhaust muffler, the exhaust sound of the exhaust gas to be discharged can be reduced by decreasing the diameter of each pipe. However, if the diameter of each pipe is too small, an engine output in a high engine speed region cannot be sufficiently drawn. Further, since the exhaust sound is small at low engine speeds in this case, it is difficult to adjust the timbre of the exhaust sound. In contrast, when the diameter of each pipe is increased, the engine output at high engine speeds can be easily sufficiently drawn. However, the sound pressure of the exhaust sound at low engine speeds becomes large.
As means for solving this problem, it has been proposed that an exhaust valve is provided so as to be switched according to engine speed, thereby making the exhaust sound and the engine output compatible with each other. However, this system is complicated in structure and costly.